The present invention relates to a method for improving data transmission in a mobile telecommunications system.
There are several multiple access modulation techniques for facilitating communications in which a large number of mobile users are present. These techniques include time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA).
In TDMA radio telecommunication systems, time division communication in the radio path takes place in successive TDMA frames each of which consists of several time slots. In each time slot, a short information packet is sent as a radio frequency burst of a finite duration, which burst consists of a number of modulated bits. For the most part, time slots are used for the transmission of control channels and traffic channels. On the traffic channels, speech and data are transmitted. On the control channels, signalling between a base station and mobile subscriber stations is carried out. The Pan-European mobile system GSM (Global System for Mobile Communications) is an example of a TDMA radio system.
CDMA is a modulation and multiple access scheme based on spread spectrum communication. Unlike FDMA or TDMA, in CDMA a large number of CDMA signals (users) simultaneously share the same wide band radio channel, typically 1.25 MHz. Pseudorandom noise (PN) binary codes, so called spreading codes, are used to distinguish between different CDMA signals, i.e traffic channels on the wide band radio channel. A separate spreading code is used over each connection between a base station and a subscriber terminal. In other words, the narrow-band data signal of the user is conventionally multiplied by the dedicated spreading code and thereby spread in bandwidth to the relatively wide band radio channel. The signals of the users can be distinguished from one another in the receivers on the basis of the unique spreading code of each connection, by using a correlator which accepts only a signal energy from the selected spreading code and despreads its spectrum into a narrow-band signal. The other users"" signals, whose spreading codes do not match, are not despread in bandwidth and as a result, contribute only to the noise and represent a self-interference generated by the system. The spreading codes of the system are preferably selected in such a way that the codes used in each system cell are mutually orthogonal, i.e. they do not correlate with each other. Thus, in the CDMA systems, the spreading code unique to each user or user"" signal provides a traffic channel in a similar sense as a time slot in the TDMA systems. CDMA is described in more detail in the document: xe2x80x9cAn overview of the application of code division multiple access (CDMA) to digital cellular systems and personal cellular networksxe2x80x9d, Qualcomm Incorporated, 1992, USA, (Document Number EX60-10010).
In traditional TDMA and CDMA mobile communications systems, the maximum data rate at the radio interface is relatively low.
For communication in conventional mobile systems, each mobile station is assigned one traffic channel for data or speech transmission. Thus, a GSM system, for example, can have as many as eight simultaneous connections to different mobile stations on a same carrier frequency. The maximum data transfer rate on a traffic channel is restricted to a relatively low level according to the bandwidth in use as well as channel coding and error correction, for example in a GSM system to 9.6 kbit/s or 12 kbit/s. In addition, in a GSM system a half-speed traffic channel (max. 4.8 kbit/s) can be chosen for low speeds of speech coding. The half-speed traffic channel is established when a mobile station communicates in a specific time slot only in every second frame, in other words, in half-speed. A second mobile station communicates in every second frame in the same time slot. This is how the capacity of the system can be doubled as far as the number of subscribers is concerned. In other words, on the same carrier wave it is possible for up to 16 mobile stations to communicate simultaneously.
In the last few years, the need for high-speed data services in mobile communication networks has remarkably increased. Data transfer rates of at least 64 kbit/s would be needed to utilize, for example, ISDN (Integrated Services Digital Network) circuit switched digital data services. PSTN data services of the public telephone network, such as modems and telefax terminals of class G3, require faster transfer rates, such as 14.4 kbit/s. One of the growing areas of mobile data transfer requiring higher transfer rates is the mobile video service. As examples of this kind of services, security control by cameras and video databases can be mentioned. The minimum data transfer rate in video transfer can be, for example, 16 or 32 kbit/s.
The data transfer rates of the present mobile communication networks are not, however, sufficient to satisfy this kind of new needs.
It is an object of the present invention to enable higher data transfer rates in mobile communication networks.
The object is achieved in a method for high-speed data transmission over a air interface between a mobile station and a fixed mobile communication network in a digital mobile communication system, this method comprising the steps of:
splitting a high-speed data signal into two or more signals of lower speed prior to a transmission over the radio path,
allocating, for high-speed data transmission, a mobile station at least two traffic channels, the number of the allocated traffic channels corresponding to the number of the signals of lower speed, and
transmitting each data signal of lower speed in different one of said allocated traffic channels.
The invention further relates to an arrangement for for high-speed data transmission in a mobile telecommunications system over the radio path between a mobile station and a base station this arrangement comprising:
means for splitting a high-speed data signal, whose speed is higher than the maximum data transfer rate of one traffic channel, into two or more signals of lower speed prior to transmission over the radio path,
a mobile station being allocated two or more traffic channels for high-speed data transmission so that each of the lower-speed signals is transmitted over the radio path in its respective traffic channel, and
means for combining the lower-speed signals received over the radio path.
The invention employs a so-called multi-channel technique so that a mobile station has access to two or more traffic channels for one data call. The high-speed data signal to be transmitted over the radio path is split into a required number of data signals of lower speed, each signal being transmitted through respective one of the allocated traffic channels. As soon as the data signals of lower speed have separately been transmitted over the radio path, they are again at the receiving end combined into the original high-speed signal. This is how the data transfer rate can be doubled, tripled, etc., depending on whether two, three or more traffic channels are assigned to be used by a subscriber. In a GSM system, for example, two traffic channels (time slots) will enable a data transfer speed of 2xc3x979.6 kbit/s which is enough for a modem of 14.4 kbit/s, or a telefax terminal, for example. Six time slots will enable a data transfer rate of 64 kbit/s.
The multi-channel technique in according to the invention, in which a high-speed data signal is transmitted as several lower-speed signals through several parallel traffic channels, has many advantages over an alternative approach in which a mobile station is assigned a single higher-capacity traffic channel having higher maximum data transmission speed than a standard traffic channel. In TDMA systems, for example, a high-speed data signal may be transmitted as several bursts in several time slots within one frame, whereas in an alternative approach in which a mobile station may be assigned several time slots in the same frame for data transmission, but the whole data signal is transmitted as one burst extended for the time of the assigned time slots. In the present invention, there is no need to change the other significant characteristics of the physical transmission path, eg. radio path and traffic channel structure. In TDMA systems, for example, these characteristics may include (at radio interface, for example, Layer 1 of GSM), such as frequency division, frame format and time slot configuration, data transfer rate, error correction, modulation, format of a TDMA burst, bit error ratio (BER), etc. In other words, the present invention allows to support different kind of subscriber data transfer rates in the radio system by a single structure of a physical transmission path. Consequently, there is no need to support several structures of a physical transmission path by the subscriber terminals, either.
By handling each of the parallel traffic channels as an independent traffic channel, a simple implementation is possible as channel coding, interleaving, burst building and modulation operations can separately be carried out to each one of the lower-speed signals. Thus, carrying out different kinds of channel codings and interleavings which depend on the required data transfer rates can be avoided. The simple embodiment is especially advantageous in cases the multi-channel technique according to the invention is applied to existing systems.
In TDMA systems, the implementation may be especially simple if adjacent time slots are employed. Consequently, it will be easier to carry out various measurements the remaining part of the frame, and increasing the number of frequency synthesizers in the receiver of the mobile station is avoided. In the GSM system, it is especially advantageous to implement the invention by two time slots.